This invention relates to casting of fine-grain metal ingots and, more particularly, to a new and improved method and apparatus for continuous casting of fine-grain ingots and to the ingots produced thereby.
For certain applications, such as components of aircraft engines and the like, it is important to obtain an ingot of metal alloy material which has a substantially uniform fine-grain structure. Efforts have been made in the past to produce fine-grain alloy ingots by various techniques. In the patents to Hunt, U.S. Pat. Nos. 4,583,580 and 4,681,787, for example, a continuous casting method is described in which the alloy to be continuously cast is heated in a cold hearth electron beam furnace and the temperature of the alloy in the hearth is controlled so as to maintain a solids content of about 15% to 40% so that the molten mixture poured from the hearth to the casting mold has a high content of solid material. As a result, the molten material in the mold has a substantially thixotropic region with a solids content of at least 50%. To maintain this condition, heat energy is applied to the material in the mold only in the region adjacent to the side wall of the mold to the extent necessary to assure the integrity of the side wall of the ingot.
To prevent hot tears in the side walls of an ingot being cast continuously, the Lowe U.S. Pat. No. 4,641,704 discloses periodic pouring of successive equal volume quantities of molten material into the mold spaced by cooling periods and intermittent lowering of the ingot in the mold following each cooling period.
A different approach described, for example, in the Hunt U.S. Pat. Nos. 4,558,729 and 4,690,875 and the Soykan et al. U.S. Pat. No. 4,261,412, utilized a mold structure into which molten drops of the ingot material fall and solidify individually with a fine-grain structure. The mold is maintained at a temperature which is below the solidus temperature of the ingot material, but above a temperature at which metallurgical bonding of the successive molten drops can occur, thereby producing an ingot without altering the size and distribution of the grains in the solidified metal drops.
Such techniques are not only complicated and difficult to execute, but also place limitations on the size and shape and properties of the resulting ingot.